1. Field of the Invention
The present invention relates to an optical recording medium such as a rewritable DVR, comprising a phase-change recording layer for high density recording.
2. Description of the Related Art
In general, recording of a binary signal and detecting of tracking signal in a compact disc (CD) or DVD is carried out utilizing a change of reflectivity caused by interference of reflected light from the bottom of a concave pit as well as a mirror finished surface. Recently, phase-change rewritable compact disc (CD-RW, or CD-Rewritable) has been generally used as the medium compatible with CD. Moreover, regarding DVD, various types of phase-change rewritable DVD have been proposed. Furthermore, it is proposed that the wavelength for recording and reproducing be shortened to 390 nm to 420 nm for a DVD capacity of 4.7 GB, which leads to increasing the numerical aperture (NA) and to realizing a DVR with the capacity of 20 GB or more (Refer to page. 210: ISOM Technical Digest 2000).
The detection of the recorded information signal in these phase-change rewritable CDs, DVDs, and DVRs is performed by using the changes of reflection rate and phase differences caused by the difference of refractive index between amorphous and crystalline states. A phase-change medium, in general, comprises a bottom protection layer, a phase-change recording layer, an upper protection layer, and a light reflection layer on a substrate. Controlling the reflection rate and phase differences by the multiple interactions of those layers makes it possible to provide the compatibility with CD and DVD. In CD-RW, it is possible to secure the compatibility of a recorded signal and channel signal in the range having a lowered reflection rate of 15% to 25%, thus enabling reproducing information in a CD drive to which an amplifier is added to cover the low reflection rate.
Since the deleting and over recording processes in phase-change recording medium can be performed solely by the modulation in strength of one converging light beam, in phase-change recording medium such as CD-RW, rewritable DVD and the like, a method of recording includes overwrite recording, in which recording and deleting are performed Simultaneously. In recording information utilizing a change of phase, it is common that rewritable phase change recording medium in practical use utilize a crystalline state as the unrecorded/deleted states, and form an amorphous mark for recording, although one of the crystalline/amorphous states and the mixed states can be used, as well as plurality of crystalline phases. For the material of the recording layer, chalcogen elements, which are chalcogenide alloys including S, Se, and Te, are mainly used.
Examples include the GeSbTe mainly comprising the GeTe—Sb2Te3 pseudobinary dual alloy, the InSbTe mainly comprising InTe—Sb2Te3 pseudobinary dual alloy, and AgInSbTe alloy and GeSnTe mainly comprising Sb0.7Te3.3 eutectic system. Among these, the types in which over-exceeding Sb is added to a GeTe—Sb2Te3 pseudobinary dual alloy, especially compositions in the neighborhood of intermetallic compounds such as Ge1Sb2Te4, Ge2Sb2Te5, and the like, have been practically used.
These compositions are characterized by crystallization which does not accompany a phase separation peculiar to an intermetallic compound. The crystal growth rate is fast, resulting in easy initialization and fast recrystallization rate on erasing. Therefore, for a recording layer providing more practical overwriting property, the related art focuses on pseudobinary dual alloy and on the compositions in the neighborhood of an intermetallic compound. (Refer to Jpn. J. Appl. Phys., vol. 69 (1991), p 2849 or SPIE, vol. 2514 (1995), pp 294–301, etc.)
Moreover, the related art has disclosed compositions of recording layers having a GeSbTe ternary composition or containing additional elements, regarding the host material of the ternary composition. Japanese Patent Application Laid-Open (JP-A) No. 61-258787, JP-A No. 62-53886, JP-A No. 62-152786, JP-A No. 01-63195, 01-211249, JP-A No. 01-277338, and so on). However, applying the materials with those compositions to optical recording medium for high density recording, such as rewritable DVD and so on, has just started and there are many problems that should be solved.